Maven acts as both a dependency management tool – it can be used to retrieve jars from a central repository or from a repository you set up – and as a declarative build tool. The difference between a “declarative” build tool and a more traditional one like ant or make is you configure what needs to get done, not how it gets done. For example, you can say in a maven script that a project should be packaged as a WAR file, and maven knows how to handle that.
Maven relies on conventions about how project directories are laid out in order to achieve its “declarativeness.” For example, it has a convention for where to put your main code, where to put your web.xml, your unit tests, and so on, but also gives the ability to change them if you need to.